Method for increasing fracture toughness and reducing brittleness of semi-crystalline polymer

ABSTRACT

A manufacturing process is provided to increase fracture toughness and reduce brittleness for a semi-crystalline polymer material. A material such as poly(vinylidene fluoride-trifluorethylene) or p(VDF-TrFE) is placed in an inert oxygen-free atmosphere and heated to a temperature that is greater than room temperature but below the melting temperature of the material. The material is then irradiated with beta particles until a desired level of fracture toughness is achieved where fracture toughness is a function of the radiation dose.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

[0001] This patent application is co-pending with one related patentapplication entitled “METHOD FOR INCREASING FRACTURE TOUGHNESS ANDREDUCING BRITTLENESS OF FERROELECTRIC POLYMERS” (Attorney Docket No.82628), by the same inventor as this patent application and file on evendate.

STATEMENT OF GOVERNMENT INTEREST

[0002] The invention described herein may be manufactured and used by orfor the Government of the United States of America for Governmentalpurposes without the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION

[0003] (1) Field of the Invention

[0004] The present invention relates generally to the manufacture ofpolymer materials, and more particularly to a method for increasingfracture toughness and reducing brittleness of a semi-crystallinepolymer material such as poly(vinylidene fluoride-trifluorethylene) orp(VDF-TrFE).

[0005] (2) Description of the Prior Art

[0006] Many semi-crystalline polymers become brittle when formed intothin sheets. In terms of a quantitative measure, these materials have alow fracture toughness which is measured as energy per unit volume inJoules/meter³ (J/m³). Brittleness is caused by a high percentage ofcrystallinity and/or an increased average size of the polymercrystallites brought about by the manufacturing process. As a result ofthe material's brittleness, damage during normal handling thereof isprevalent thereby increasing the cost of using semi-crystalline polymersin various products.

[0007] In some applications, crystallinity percentages in excess of 80%are desired or required in order for the semi-crystalline material toperform properly. For example, in order to optimize the performance ofcertain electroactive polymers, it is necessary to anneal the materialto a very high level of crystallinity. However, while the annealing stepgreatly increases the material's crystallinity in preparation for aferroelectric poling operation, this processing step also makes thetreated material so brittle that it often cracks during routine handlingthereof.

SUMMARY OF THE INVENTION

[0008] Accordingly, it is an object of the present invention to providea method for increasing a semi-crystalline polymer's fracture toughnessto thereby reduce its brittleness.

[0009] Another object of the present invention is to provide a methodthat reduces the costs associated with using brittle semicrystallinepolymer materials.

[0010] A still further object of the present invention is to provide forincreased use of semi-crystalline polymer materials in applicationswhere the material's brittleness previously prevented such use.

[0011] Other objects and advantages of the present invention will becomemore obvious hereinafter in the specification and drawings.

[0012] In accordance with the present invention, a method is providedfor increasing fracture toughness and reducing brittleness of asemi-crystalline polymer material. The material is placed in an inertoxygen-free atmosphere and heated to a temperature that is greater thanroom temperature but below the melting temperature of the material. Thematerial is then irradiated with beta particles until a desired level offracture toughness is achieved. Fracture toughness is a function of theradiation dose received by the material. Specific processing steps areprovided for the semi-crystalline poly(vinylidenefluoride-trifluorethylene) or p(VDF-TrFE).

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Other objects, features and advantages of the present inventionwill become apparent upon reference to the following description of thepreferred embodiments and to the drawings, wherein correspondingreference characters indicate corresponding parts throughout the severalviews of the drawings and wherein:

[0014]FIG. 1 is a schematic view of an apparatus for carrying out themethod of increasing fracture toughness of a semi-crystalline polymermaterial in accordance with the present invention;

[0015]FIG. 2 is a graph of fracture toughness as a function of radiationdosage for the semi-crystalline polymer poly(vinylidenefluoride-trifluorethylene) after processing in accordance with thepresent invention; and

[0016]FIG. 3 is a graph of fracture toughness as a function of radiationdosage for the annealed ferroelectric polymer poly(vinylidenefluoride-trifluorethylene) after processing in accordance with thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

[0017] Referring now to the drawings, and more particularly to FIG. 1, asystem for carrying the method of increasing fracture toughness inaccordance with the present invention is shown and referenced generallyby numeral 10. As is known in the art, fracture toughness is aquantitative measurement indicative of the energy required tocrack/fracture a material. Thus, increasing fracture toughness of amaterial reduces its brittleness which, while not a measurable quantity,describes a quality thereof.

[0018] System 10 includes a fixture or chamber 12 (e.g., a sealedchamber, irradiation fixture, etc.) for holding a semi-crystallinepolymer material 14 to be processed in accordance with the presentinvention. In order to assure that no chemical reactions occur betweenmaterial 14 and its surrounding gaseous environment, an inert gas source16 supplies chamber 12 with an inert gas environment, i.e., inert withrespect to material 14. Typically, a flow of inert gas is passedcontinuously through chamber 12 as indicated by arrows 18. The gas isoxygen-free because many polymeric materials can react with oxygenduring irradiation thereof. Although not an exhaustive list, some commoninert gases suitable for use in the present invention include nitrogenand argon.

[0019] A heater 20 is coupled to chamber 12 for raising the temperatureof material 14 during the processing thereof. A radiation source 22generates a beam of beta radiation that is directed to/through chamber12, i.e., material 14 is exposed to high energy electrons. A radiationdosage monitor 24 is coupled to/through chamber 12 formonitoring/measuring the radiation dosage to which material 14 isexposed. System 10 so constructed/configured can be made from a varietyof commercially-available components as would be understood by one ofordinary skill in the art.

[0020] In operation of system 10 in accordance with the presentinvention, material 14 is placed in chamber 12 and a flow 18 of inertgas is provided to chamber 12 by inert gas source 16. Flow 18 should besufficient to purge chamber 12 of oxygen-containing atmospheric gas sothat only the inert gas is contained in chamber 12. Heater 20 isactivated to heat material 14 to a temperature that, in general, isgreater than room temperature (i.e., approximately 20-25° C.) but belowthe melting temperature of material 14. In the present invention, withmaterial 14 being heated under an inert gas purge, radiation source 22irradiates material 14 with beta particles of a specified energy.Radiation dosage is simultaneously monitored by a radiation monitor 24which is representative of direct monitoring systems or indirectmonitoring systems such as those capable of monitoring electron flux. Inthese conditions, it was found that an increase in fracture toughnesswas associated with the energy level of the radiation and the amount ofradiation to which material 14 is exposed. Thus, depending on the typematerial 14 and the desired level of fracture toughness, irradiation ofmaterial 14 continues until a specified radiation dosage is achievedindicative of the desired level of fracture toughness for a given energylevel of radiation.

[0021] The above-described process was used successfully for thesemi-crystalline polymer poly(vinylidene-trifluorethylene) (orp(VDF-TrFE)) comprised of 50-85 weight percent vinylidene fluoride (VDF)which, in its cast form, has a crystallinity of 40-50%. However,crystallinity of this material can be increased to 80-90% or more usingan annealing process. Whether cast or annealed, p(VDF-TrFE) is generallyformed into thin sheets which are less brittle in the cast form, buthighly brittle in the annealed form.

[0022] To increase the fracture toughness of semi-crystallinep(VDF-TrFE) in accordance with the present invention, the p(VDF-TrFE)material was placed in an irradiation fixture under a nitrogen purge,and heated to a temperature between approximately 100-120° C., i.e.,greater than room temperature but below the melting point ofp(VDF-TrFE). The p(VDF-TrFE) was then irradiated with approximately 1.2mega electron volt (MeV) beta particles while the radiation wasmonitored. A graph of the resulting fracture toughness as a function ofradiation dosage is illustrated in FIG. 2 for a p(VDF-TrFE) polymercomprised of 65 weight percent VDF that was heated to 100° C. under anitrogen purge.

[0023] As is evident from the graph in FIG. 2, significant improvementsin fracture toughness were realized. The greatest increase in fracturetoughness occurred when radiation dosage was increased fromapproximately 60 to approximately 80 megarads (Mrads) for the indicatedradiation dosage. Note that this result is unexpected since, below themelting temperature, beta particle radiation on its own would beexpected to induce polymer chain scissioning and perhaps somecross-linking, both of which tend to reduce fracture toughness. It istherefore believed that the present invention's combination of stepsincreases fracture toughness by means of a reduction in the average sizeof the crystallites in the material through the generation of pendantgroup defects which interfere with crystallinity.

[0024] It is apparent from the above description that thesemi-crystalline polymer p(VDF-TrFE) can undergo dramatic increases infracture toughness in accordance with the present invention. However,p(VDF-TrFE) is also commonly used in its ferroelectric state for themanufacture of transducers and hydrophones. To achieve the ferroelectricstate, cast semi-crystalline p(VDFTrFE) can be poled to align itsdomains, or can first be annealed to increase crystallinity and thenpoled as is well known in the art. The problem that plagues cast orannealed ferroelectric p(VDF-TrFE) is its brittleness. However, for suchuses, any increase in fracture toughness/decrease in brittleness must beachieved while maintaining the material's ferroelectricproperties/domains. Unfortunately, it was discovered that theabove-described process of increasing fracture toughness caused adestruction of ferroelectric domains at levels of fracture toughnessthat were less than 1 J/m³. That is, the dramatic increases in fracturetoughness evidenced in FIG. 2 came at the expense of ferroelectricdomain destruction in the case of ferroelectric p(VDF-TrFE).

[0025] To overcome this problem, it is necessary to select a suitablebeta particle radiation energy level and radiation dosage that achievesa desired level of fracture toughness without destroying the material'sferroelectric properties. In terms of cast or annealed ferroelectricp(VDF-TrFE) comprised of 50-85 weight percent VDF, it was found that anincreased radiation energy level could increase the material's fracturetoughness without destroying its ferroelectric properties. This isevidenced in FIG. 3 for an annealed ferroelectric p(VDF-TrFE) having 65weight percent VDF that is heated to 120° C. under a nitrogen purgeprior to irradiation with 2.55 (MeV) beta particles. In particular, itwas found that at this radiation energy level, ferroelectric propertieswere substantially maintained for radiation doses up to approximately 50Mrads. After this, increased fracture toughness came at the expense ofdestroyed ferroelectric properties. Note that substantial fracturetoughness was achieved and ferroelectric properties maintained at aradiation dosage of approximately 32.5 Mrads. Also, note that if onlyincreased fracture toughness is of concern, this example implies thatfracture toughness can be increased by utilizing beta particles havingan energy level between approximately 1.0-3.0 MeV.

[0026] The advantages of the present invention are numerous. Dramaticincreases in fracture toughness are achieved for the semi-crystallinepolymer p(VDF-TrFE). While the mechanisms at work in the presentinvention are not fully understood, it is believed that theabove-described methodology can be extended to other semi-crystallinepolymers. In general, once a desired fracture toughness is identified, aparticular combination of heating, electron energy bombardment andradiation dosage can be implemented in a repeatable manufacturingprocess. With the heat and electron energy being fixed for a givenmaterial, only radiation dosage need be monitored as fracture toughnessis a function thereof in the present process.

[0027] It will be understood that many additional changes in thedetails, materials, steps and arrangement of parts, which have beenherein described and illustrated in order to explain the nature of theinvention, may be made by those skilled in the art within the principleand scope of the invention as expressed in the appended claims.

What is claimed is:
 1. A method for increasing fracture toughness andreducing brittleness of a semi-crystalline polymer material, comprisingthe steps of: placing a semi-crystalline polymer material in anoxygen-free atmosphere that is inert with respect to said material;heating said material to a temperature that is greater than roomtemperature and below the melting temperature of said material; andirradiating said material so-heated with beta particles until a desiredlevel of fracture toughness is achieved, wherein said fracture toughnessis a function of a radiation dose received by said material.
 2. A methodaccording to claim 1 wherein said atmosphere is a gas selected from thegroup consisting of nitrogen and argon.
 3. A method according to claim 1wherein said material is poly(vinylidene fluoride-trifluorethylene), andwherein the temperature to which said material is heated is betweenapproximately 100-120° C.
 4. A method according to claim 1 wherein saidmaterial is poly(vinylidene fluoride-trifluorethylene) havingapproximately 50-85 weight percent vinylidene fluoride.
 5. A methodaccording to claim 1 wherein said material is poly(vinylidenefluoride-trifluorethylene) having approximately 65 weight percentvinylidene fluoride, and wherein the temperature to which said materialis heated is approximately 100° C.
 6. A method according to claim 5wherein said beta particles have an energy level of betweenapproximately 1.0-3.0 mega electron volts (MeV).
 7. A method accordingto claim 1 wherein said material is poly(vinylidenefluoride-trifluorethylene), and wherein said beta particles have anenergy level of between approximately 1.0-3.0 mega electron volts (MeV).8. A method for increasing fracture toughness and reducing brittlenessof semi-crystalline poly(vinylidene fluoride-trifluorethylene),comprising the steps of: providing an environment containing a gasselected from the group consisting of nitrogen and argon; placing asemi-crystalline poly(vinylidene fluoride-trifluorethylene) material insaid environment; heating said material to a temperature that isapproximately between 100-120° C.; irradiating said material so-heatedwith beta particles; and monitoring a radiation dose received by saidmaterial during said step of irradiating until a desired level offracture toughness is achieved, wherein said fracture toughness is afunction of said radiation dose.
 9. A method according to claim 8wherein said material is approximately 50-85 weight percent vinylidenefluoride.
 10. A method according to claim 8 wherein said material isapproximately 65 weight percent vinylidene fluoride, and wherein thetemperature to which said material is heated is approximately 100° C.11. A method according to claim 10 wherein said beta particles have anenergy level of between approximately 1.0-3.0 mega electron volts (MeV).12. A method according to claim 8 wherein said beta particles have anenergy level of between approximately 1.0-3.0 mega electron volts (MeV).13. A method for increasing fracture toughness and reducing brittlenessof annealed poly(vinylidene fluoride-trifluorethylene), comprising thesteps of: providing an environment containing a gas selected from thegroup consisting of nitrogen and argon; placing an annealedpoly(vinylidene fluoride-trifluorethylene) material in said environment;heating said material to a temperature that is greater than roomtemperature and below the melting temperature of said material;irradiating said material so-heated with beta particles; and monitoringa radiation dose received by said material during said step ofirradiating until a desired level of fracture toughness is achieved,wherein said fracture toughness is a function of said radiation dose.14. A method according to claim 13 wherein said material isapproximately 50-85 weight percent vinylidene fluoride.
 15. A methodaccording to claim 13 wherein the temperature achieved to which saidmaterial is heated is between approximately 100-120° C.
 16. A methodaccording to claim 15 wherein said beta particles have an energy levelof between approximately 1.0-3.0 mega electron volts (MeV).
 17. A methodaccording to claim 13 wherein said beta particles have an energy levelof between approximately 1.0-3.0 mega electron volts (MeV).